Fuel cells offer the opportunity of obtaining electric power with high efficiency from electro chemical conversion of hydrogen. However, as hydrogen is difficult to store or transport because of the high explosion hazard associated with it, methanol or hydrocarbons are, at present, used as the hydrogen source and hydrogen is then produced from these compounds in an upstream reformer. Methanol is liquid under normal conditions and can therefore be transported and stored without any great problems. Hydrocarbons are either likewise liquid under normal conditions or can easily be liquefied under super atmospheric pressure. In the case of natural gas, which consists essentially of methane, an appropriate infrastructure already exists, so that stationary energy-producing apparatus based on fuel cells can readily be operated using methane as the starting material.
Hydrogen can be liberated from methane by steam reforming. The resulting gas consists essentially of hydrogen, carbon dioxide and carbon monoxide together with traces of unreacted methane and water. This gas can be used as fuel gas for a fuel cell. To shift the equilibrium in steam reforming to the side of hydrogen, reforming has to be carried out at temperatures of about 650 degrees C. To achieve a constant composition of the fuel gas, this temperature should be adhered to as exactly as possible.
Fuel cell arrangements in which the fuel gas produced from methane and water can be utilized for generation of energy are known in the art. Such arrangements may comprise a number of fuel cells which are arranged in a fuel cell stack within a closed protective housing. Fuel gas, consisting essentially of hydrogen, carbon dioxide, carbon monoxide and residual methane and water, is fed to the fuel cells via an anode gas inlet. The fuel gas is produced from methane and water in the upstream reformer. On the anode side, the fuel gas is consumed to produce electrons according to the following reaction equations:CO32−+H2→H2O+CO2+2e−CO32−+CO→2CO2+2e−
To achieve high efficiency of the fuel cell, the reaction is carried out so that it does not proceed to completion. The anode waste gas therefore comprises not only the reaction products, carbon dioxide and water, but also unconverted hydrogen, carbon monoxide and methane. To remove the residual hydrogen, carbon monoxide and methane the anode waste gas is mixed with air and then fed to a catalytic waste gas burner in which the remaining methane, carbon monoxide and traces of hydrogen are oxidized to form water and carbon dioxide.
To remove residues of hydrogen, therefore, the anode waste gas is first mixed with air and then fed to a catalytic waste gas burner in which the remaining methane, carbon monoxide and also traces of hydrogen are oxidized to water and carbon dioxide. Optionally, or alternatively, in addition to the anode waste gas and air, other gases such as cathode waste gas can be admixed. The thermal energy released in the process can be used in different ways.
In the prior art noble metals, for example platinum and/or palladium, which are provided in finely-distributed form on a suitable support, are currently used as catalysts for waste gas burners. This catalytic combustion has the advantage that it is very steady and has no temperature peaks. The oxidation reaction on palladium catalysts proceeds at temperatures in the range from approximately 450 to 550° C. At higher temperatures of over approximately 800 to 900° C., the Pd/PdO balance shifts in favor of palladium metal, whereby the activity of the catalyst decreases. A loss of activity is observed as a result of sintering occurring or the coking of the catalyst particles. In addition, noble metal catalysts have the disadvantage of very high raw material prices.
Alternatively, heat-stable catalysts for the catalytic combustion of methane are known. These are based on alkaline earth hexaaluminates which contain Mn, Co, Fe, Ni, Cu or Cr. These catalysts are characterized by high activity and resistance, even at temperatures of more than 1200° C. However, the activity of these catalysts is relatively low at temperatures in the preferred range of 500-800° C. To be able to provide adequate catalytic activity also at lower temperatures, small quantities of platinum metals are added, for example Pt, Ru, Rh or Pd.
Notwithstanding, there is still a need for a cost favorable, active catalyst for long term stability for fuel cell arrangement which comprises a catalytic waste gas oxidizer for the oxidation of a mixture of anode waste gases, including CO, H2 and CH4, air and optionally other gases, such as cathode waste gases, which is stable and active for the removal of methane, CO and H2.